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If you own a Surface Pro 6 or Surface Book 2 and you’ve noticed slower-than-expected performance lately, we’ve got good news: You aren’t imagining it. We’ve also got bad news: Currently, there is no fix, though Microsoft is reportedly working on one. There have been reports on Reddit from users who saw their CPU speeds falling to as low as 400MHz, even on systems plugged into the wall. What makes this issue a little curious, however, is that some of these reports go back for several years.

TechRepublic contacted Microsoft, who said: “We are aware of some customers reporting a scenario with their Surface Books where CPU speeds are slowed,” a Microsoft spokesperson told TechRepublic. “We are quickly working to address via a firmware update.”

While this quote doesn’t reference the issue with Surface Pro, some of the Reddit threads focus on that platform rather than the Surface Book or Surface Book 2. In this case, however, it appears that the issue has been flying along under the radar for a while — long enough that people have found their own fix. There’s a third-party utility called ThrottleStop. It can be used to disable a specific CPU instruction called BD PROCHOT.

What’s a BD PROCHOT?

BD PROCHOT stands for “Bidirectional processor hot.” ‘Bidirectional’ refers to the fact that this signal can be thrown by the CPU itself (to lower its own temperature) or by another component as a means of protecting that part from overheating. Imagine, for example, that your GPU was going to overheat and damage itself due to high operating temperature. The GPU can throw this flag to reduce the CPU’s frequency, reducing the processing work that was causing an overheat in the first place.

It is not clear exactly which components can use the BD PROCHOT function. It was introduced at a time when individual components paid less attention to their own thermal conditions and performed less clock throttling overall, though that intelligence has been baked into mobile hardware at various times and in various ways. Some modern systems measure skin temperature and use it as part of determining their operating frequencies, for example.

Because BD PROCHOT is bidirectional, there’s a potential risk here. The flag can be thrown by a different component that isn’t the CPU, which means (in theory), you’re overriding a component that’s trying to preserve its own functionality by telling the CPU to slow down. But this does not seem to be an explanation for what’s going on with Surface devices. Customers report these problems on products that show no signs of any overheat, and some customers who have used the ThrottleStop utility to turn the BD PROCHOT function off have gone on to use their products for a year or more without any problems.

There are non-Surface users who have reported BD PROCHOT issues as well over the years, so this isn’t a Microsoft-specific instruction or capability, though the issue does seem to be hitting Surface in particular right now. Users who need to use ThrottleStop to restore normal functionality can probably attempt doing so. If you do, be cautious. Don’t disable BD PROCHOT and then run the ugliest thermal workload you can find — turn the flag off and let the system sit at idle or run a YouTube video. See if you notice any inappropriate behavior or hot spot formation on the device. If you run into problems, you can use the utility again to restore default functionality.

Devices like the Surface Book and Surface Pro 6 are thin, which means any significant source of inappropriate heat is likely to migrate to the surface (no pun intended) of the device. So before you disable BD PROCHOT, if you’re concerned about the impact, take a moment to familiarize yourself with how hot your Surface Book or Surface Pro 6 feels to the touch now. Then disable BD PROCHOT and run the same check after giving the system time to warm up. If the machine stays stable and you have no other problems, you’re probably fine to use it. Just keep an eye out for a fix from Microsoft.

If your machine feels hotter to the touch than it ought to, or the heat is building up in an unusual spot for your machine, we advise against disabling PROCHOT until more information is available. People reporting this issue seem to be experiencing a bug rather than a genuine problem but that is not a guarantee. If you experience any instabilities or thermal issues after disabling BD PROCHOT via third-party utility, re-enable it immediately and contact Microsoft for additional service instructions.

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